1. The Field of the Invention
The invention generally relates to an electric lamp. The invention specifically relates to a light-transmitting lamp vessel, which accommodates a light source for emitting a visible light and has a light-absorbing coating for absorbing part of the visible light.
2. Description of the Related Art
Such electric lamps are predominantly used as indicator lamps in vehicles, for example as an amber-colored light source in indicators or as a red-colored light source in brake lights of automobiles. Alternative embodiments of such lamps, wherein the color temperature is increased by a light-absorbing coating, can also be used as headlamps of a vehicle. The light-absorbing coatings are also used as a color layer on (incandescent) lamps for general lighting purposes. The electric lamps can also be used in traffic lights.
A known electric lamp disclosed in Canadian Patent No. CA-A 0 766 196 has a coating applied to the lamp vessel, which coating comprises a substance which absorbs visible light, for example a dye and/or a pigment.
For the application of the coatings, use is generally made of organic lacquers. The organic lacquer forms a kind of carrier matrix containing the pigment or the dye. The organic lacquer enables, inter alia, a good adhesion of the coating to the lamp vessel to be obtained. In the known lamp, use is made of a polymethylmethacrylate polymer, which is applied to the lamp vessel by a dip coating. In an alternative embodiment, a lacquer of a polyester silicone is applied to the lamp vessel by a spraying process. Moreover, use is often made of organic dyes, such as a dye called Zapon 157. Such dyes are added to the lacquer layer to obtain the desired color point.
It is a drawback of the known electric lamp comprising a light-absorbing coating on the basis of an organic lacquer that the adhesion of the coating to the lamp vessel deteriorates substantially and/or the organic dye degrades at temperatures above approximately 220xc2x0 C. At temperatures close to or higher than the temperature, there is an increased risk that the coating cracks and/or becomes detached from the lamp vessel. Since the dimensions of the luminaires accommodating the electric lamp decrease continuously as do the dimensions of the electric lamp itself, the temperature of the lamp vessel provided with the coating currently reaches a temperature of 250xc2x0 C. In addition, there is a trend towards further miniaturization, so that the lamp vessel provided with the light-absorbing coating reaches temperatures of approximately 325xc2x0 C.
It is an object of the invention to provide an electric lamp to address the above drawbacks of the prior art.
In accordance with the invention, an electric lamp is characterized in that a light-absorbing coating comprises a network which can be obtained by conversion of an organically modified silane by a sol-gel process, and the organically modified silane is selected from the group formed by compounds of the following structural formula: Rxe2x80x2Si(ORxe2x80x3)3, wherein Rxe2x80x2 comprises an alkyl group or an aryl group, and wherein Rxe2x80x3 comprises an alkyl group.
By replacing the organic lacquer in the light-absorbing layer in the known electric lamp by a network comprising an organically modified silane as the starting material, an optically transparent, non-scattering, light-absorbing coating is obtained which can resist temperatures up to 400xc2x0 C. By using an organically modified silane in the manufacture of the network, a part of the Rxe2x80x2 groups, i.e. the alkyl or aryl groups, remain present as an end group in the network. As a result, instead of four network bonds per Si atom, the network in accordance with the invention has fewer than four network bonds per Si atom. This results, by way of example, in a network comprising, on average, approximately three network bonds per Si atom. Despite the fact that the network is partly composed of the alkyl or aryl groups, a network is obtained whose density is at least substantially equal to that of the customary silica network. Unlike the customarily used silica network, a network which is partly composed of the alkyl or aryl groups has a greater elasticity and flexibility. This enables relatively thick light-absorbing coatings to be manufactured.
Preferably, the Rxe2x80x2 group comprises CH3 or C6H5. These substances have a relatively good thermal stability. A network comprising methyl or phenyl groups enables thicker coatings to be obtained. Experiments have further shown that coatings wherein methyl or phenyl groups are incorporated in a network are stable up to a temperature of at least 350xc2x0 C. the groups are end groups in the network and remain part of the network at the higher temperatures. At such a relatively high temperature load on the light-absorbing coating, no appreciable degradation of the network occurs during the service life of the electric lamp.
Preferably, the Rxe2x80x3 group comprises CH3 or C2H5. Methyl and ethyl groups are particularly suitable because methanol and ethanol are formed in the hydrolysis process, which substances are compatible with the pigment dispersion and evaporate relatively easily. In general, the methoxy groups (xe2x80x94OCH3) react more rapidly than the ethoxy groups (xe2x80x94OC2H5) which, in turn, react more rapidly than (iso)propoxy groups (xe2x80x94OC3H7). For a smooth hydrolysis process, use is advantageously made of Rxe2x80x3 groups which are not very long.
Very suitable starting materials for the manufacture of the network in accordance with the invention are (1) methyltrimethoxy silane (MTMS), where Rxe2x80x2=Rxe2x80x3=CH3, (2) methyltriethoxy silane (MTES), where Rxe2x80x2=CH3 and Rxe2x80x3=C2H5, (3) phenyltrimethoxy silane (PTMS), where Rxe2x80x2=C6H5 and Rxe2x80x3=CH3, and
(4) phenyltriethoxy silane (PTES), where Rxe2x80x2=C6H5 and Rxe2x80x3=C2H5. Such starting materials are known per se and commercially available.
An embodiment of the electric lamp in accordance with the invention is characterized in that the thickness tc of the light-absorbing coating is tcxe2x89xa71 xcexcm. If use is made of a network composed of silica, which comprises four network bonds per Si atom, the thickness of the coating is limited, under atmospheric conditions, to approximately at most 0.5 xcexcm. In such silica layers whose thickness exceeds the thickness, stress in the layer readily leads to cracks and/or the coating readily becomes detached from the lamp vessel. By using a network comprising fewer than four network bonds per Si atom, a much thicker layer thickness can be attained. Preferably, tcxe2x89xa72 xcexcm. In thicker, light-absorbing coatings, more pigment or dye can be incorporated, whereby the color effect of the coating is improved.
Inorganic filling materials may be incorporated in the light-absorbing coating. For this purpose, in a favorable embodiment of the electric lamp in accordance with the invention, silica particles having a diameter dxe2x89xa750 nm are incorporated in the network. Incorporation of these so-called nano-sized silica particles reduces shrinkage of the layer during the manufacture thereof. In addition, the incorporation of the nano-sized silica particles makes it possible to obtain even thicker coatings which bond well to the lamp vessel. By adding nano-sized silica particles to a network, wherein alkyl or aryl groups, which form the Rxe2x80x2 groups, are present as the end group, 20 xcexcm thick layers having favorable bonding properties can be obtained. Such thick layers can contain considerable quantities of a pigment or a dye to obtain the desired color point of the light-absorbing coating. By incorporating the silica particles it becomes possible to manufacture light-absorbing coatings in a larger thickness. The refractive index of such a coating is less influenced by the refractive index of the pigment when the same quantity of pigment is incorporated in a thicker coating. The use of the silica particles thus results in a certain degree of freedom to bring the refractive index of the light-absorbing coating to a desired value and maintain the refractive index at the value.
To manufacture light-absorbing coatings having the desired optical properties, the coatings having the desired thermal stability during the service life of the electric lamp, use is preferably made of inorganic pigments. In a favorable embodiment of the electric lamp in accordance with the invention, the pigment is selected from the group formed by iron oxide, iron oxide doped with phosphor, zinc-iron oxide, cobalt aluminate, neodymium oxide, bismuth vanadate, zirconium praseodymium silicate or mixtures thereof. Iron oxide (Fe2O3) is an orange pigment and P-doped Fe2O3 is an orange-rd pigment. Zinc-iron oxide, for example ZnFe2O4 or ZnO.ZnFe2O4 are yellow pigments. Mixing (P-doped) Fe2O3 with ZnFe2O4 yields a pigment of a deep orange color. Cobalt aluminate (CoAl2O4) and neodymium oxide (Nd2O5) are blue pigments. Bismuth vanadate (BiVO4), also referred to as pucherite, is a yellow-green pigment. Zirconium praseodymium silicate is a yellow pigment. Experiments have shown that a network including the inorganic pigments does not appreciably degrade during the service life and at the relatively high temperature load on the light-absorbing coating.
In an alternative embodiment, light-absorbing coatings are obtained wherein organic pigments are used. Particularly suitable pigments are the so-called Red 177 (anthraquinone) or chromium phthalic yellow (2RLP) from xe2x80x9cCibaxe2x80x9d. Further suitable pigments are Red 149 (perylene), Red 122 (quinacridone), Red 257 (Ni-isoindoline), Violet 19 (quinacridone), Blue 15:1 (Cu-phthalocyanine), Green 7 (hal.Cu-phthalocyaninc) or Yellow 83 (dyaryl) from xe2x80x9cClariantxe2x80x9d. Also mixtures of inorganic and organic pigments are suitable, for example a mixture of chromium phthalic yellow and (zinc)iron oxide.
Preferably, an average diameter dp of the pigment particles is dpxe2x89xa6100 nm. By using pigments of such relatively small dimensions, optically transparent coatings are obtained which exhibit relatively little light scattering. Since the electric lamp in accordance with the invention is often applied in specially designed reflectors, wherein the light source is embodied so as to be punctiform, light scattering by the light-absorbing coatings is an undesirable property. The effect of light scattering is at least substantially precluded if the average diameter of the pigment particles dpxe2x89xa650 nm.
In the literature, networks obtained by conversion of an organically modified silane are customarily used to manufacture light-scattering coatings. In this invention, however, the network is used, in particular, to manufacture transparent coatings exhibiting relatively little light scattering.
Particularly suitable electric lamps are obtained by applying a pigment in a light-absorbing coating, which pigment is composed of a mixture of iron oxide and bismuth vanadate, or of a mixture of iron oxide doped with phosphor and bismuth vanadate. Since bismuth vanadate often is only available in a particle size dp, where dp greater than 100 nm, a light-absorbing coating comprising such a pigment often exhibits a disturbing degree of light scattering. Inventors have found in experiments that the use of a combination of (P-doped) iron oxide and bismuth vanadate as the pigment causes the light scattering of the coating obtained to be reduced considerably as if the diameter of the bismuth vanadate particles is much smaller than 100 nm. Without being obliged to give any theoretical explanation, the decrease of the light scattering of such a coating is attributed to an increase of the refractive index of the network as a result of the presence of the iron oxide particles.
It has been found that an electric lamp comprising a lamp vessel which is coated in accordance with the invention with a light-absorbing coating comprising a network obtained by conversion of an organically modified silane by a sol-gel process preserves its initial properties to a substantial degree during the service life of the electric lamp.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.